1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, and more particularly to a disk with a perpendicular magnetic recording layer for use in magnetic recording hard disk drives.
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. The most common type of perpendicular magnetic recording system is one that uses a “probe” or single pole recording head with a “dual-layer” media as the recording disk, as shown in FIG. 1. The dual-layer media comprises a perpendicular magnetic data recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL), with the SUL serving as a flux return path for the field from the pole recording head. In FIG. 1, the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits.
FIG. 2 is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field Hw acting on the recording layer RL. The disk also includes the hard disk substrate, a seed or onset layer (OL) for growth of the SUL, an exchange-break layer (EBL) to break the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and to facilitate epitaxial growth of the RL, and a protective overcoat (OC). As shown in FIG. 2, the RL is located inside the gap of the “apparent” recording head (ARH), which allows for significantly higher write fields compared to longitudinal or in-plane recording. The ARH comprises the write pole (FIG. 1) which is the real write head (RWH) above the disk, and an effective secondary write pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which is decoupled from the RL by the EBL and by virtue of its high permeability produces a magnetic mirror image of the RWH during the write process. This effectively brings the RL into the gap between the RWH and the SWP and allows for a large write field Hw inside the RL.
One type of material for the RL is a conventional granular cobalt alloy, such as a CoPtCr alloy. This conventional material has out-of-plane perpendicular magnetic anisotropy as a result of the c-axis of its hexagonal-close-packed (hcp) crystalline structure being induced to grow perpendicular to the plane of the layer during deposition. To induce this growth, the EBL onto which the RL is formed is also typically a material with an hcp crystalline structure. Thus ruthenium (Ru) is one type of material proposed for the EBL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity media and to reduce inter-granular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the RL has been proposed by the addition of metal oxides, including oxides of Si, Ti and Ta, which precipitate to the grain boundaries.
A perpendicular magnetic recording medium has been proposed wherein the RL is an antiferromagnetically-coupled (AFC) recording layer of two ferromagnetic layers, each having perpendicular magnetic anisotropy, separated by a coupling layer that mediates antiferromagnetic (AF) coupling. In this type of medium, as described in U.S. 6,815,082 B2, both the first or lower ferromagnetic layer and the second or upper ferromagnetic layer are formed of a conventional granular cobalt alloy. Thus in a perpendicular magnetic recording medium with an AFC RL, the EBL would also have to have an hcp crystalline structure to induce the perpendicular magnetic anisotropy of the lower layer in the AFC RL. The AF-coupling layer induces perpendicular antiferromagnetic exchange coupling between the two ferromagnetic layers, as depicted in FIG. 3 by the antiparallel magnetization directions between the two ferromagnetic layers in each magnetized region of the AFC RL. The upper ferromagnetic layer is formed with a higher magnetic moment than the lower ferromagnetic layer, typically by making it thicker, so that the AFC RL has a net magnetic moment in the absence of a magnetic field.
The best performance for writing perpendicular magnetic recording media is obtained when the EBL is as thin as possible, i.e., the minimum thickness required to provide magnetic decoupling of the SUL and the RL, so that flux can readily pass through the EBL during the write process. However, while a reduction in thickness of the EBL is desirable, there are other reasons why the EBL has a certain thickness. First, the EBL should be thick enough to provide the template for the growth of the cobalt alloy RL to cause its c-axis to be perpendicular. A relatively thick Ru EBL also provides an RL with high coercivity and low enough inter-granular exchange coupling to minimize the intrinsic media noise. Thus, if Ru is used as the EBL it should be at least approximately 80 Å thick for current RL materials. An additional reason for a relatively thick EBL is given by the fact that the SUL also affects the read-back amplitude of the magnetic transitions as read by the read element or head. In particular, low-frequency transitions have much higher amplitudes. The thinner the EBL the greater is the amplitude increase at low frequency. Therefore, when the EBL is too thin the dynamic amplitude range that the read head needs to be sensitive to is quite large. This makes design of the read head and associated read circuitry very challenging.
What is needed is a perpendicular magnetic recording medium with an effective EBL, that is or appears thin during the write process for maximum write field enhancement, but is or appears thicker during the readback process to limit the low field signal amplitude and the dynamic range of the read sensor.